This invention relates to an apparatus for liquid selective etching of a substrate, e.g. silicon, body and to a method of performing that etching.
Single crystal transducer structures, e.g. of selectively etched silicon, are finding increasing use in instrumentation applications where their ruggedness and small size are of particular advantage. These applications include the measurement of pressure, temperature and accelerational forces. The transducer structures comprise a unitary body formed by selective etching from a substrate body or blank of single crystal material. Conveniently, silicon is used for this purpose as it has excellent mechanical properties and is not unduly sensitive to changes in temperature. Typical examples of etched silicon transducers are described in our published UK specifications Nos. 2,130,373 and 2,139,358.
In a typical fabrication process, the transducer devices are configured in a substrate body either by means of a photolithographic mask or by selective boron doping, and a defined by exposure of the substrate to a selective anisotropic etch. Generally, potassium hydroxide (KOH) etches are employed, a typical etch comprising a mixture of, water, potassium hydroxide and isopropyl alcohol.
Prior to the present application, etching of transducer devices have been performed as a `one off` process or individual substrate wafers. Attempts to adapt this process to the simultaneous etching of a plurality of wafers have met with a number of difficulties. In particular, lack of uniformity of etching prevents the strict process control that is necessary for volume manufacture. Also, it has been found that the process can suffer from a phenomenon known as pyramid formation. Pyramidal structures of unetched material extend from the etched surfaces of the substrate body and reduce the yield of usable devices. Furthermore, it has been found that boron etch stops are not always effective with conventional etching processes.
The object of the invention is to minimise or to overcome these disadvantages.